Attention Deficit Hyperactivity Disorder (ADHD) is a complex behavioral condition characterized, in part, by distractibility, impulsivity, hyperactivity, and abnormal novelty-seeking behavior. ADHD is currently estimated to affect 2.5 million children and adults nationwide. The molecular basis for ADHD is unknown but a wealth of clinical data supports the hypothesis that dysregulation of dopamine (DA) signaling in the central nervous system significantly contributes to its etiology. Although most individuals diagnosed with ADHD benefit from low doses of the psychostimulant Ritalin (r) (methylphenidate, MPD), its exact mode of action and long-term consequences of exposure are unclear. However, since MPD can elevate extracellular DA levels by interfering with DA transporter function, its cIinical benefit may involve an indirect stimulation of DA receptors (DARs). Of the five known DAR subtypes, recent family and twin studies have revealed an association between an allele of the human DA D4R gene (DRD4.7) and ADHD. Anatomically, D4Rs are expressed in brain regions thought to be relevant to ADHD. Furthermore, incipient congenic D4R-/-mice display locomotor supersensitivity to the psychostimulants cocaine and methamphetamine in addition to elevated striatal dopamine content and diminished novelty-seeking behavior. Taken together these observations suggested to us the hypothesis: DA D4Rs mediate MPD's effects in mice and humans. Since the selective pharmacological antagonism of D4Rs in vivo has yet to be convincingly demonstrated, we chose to begin testing our hypothesis in D4R-deficient mice and have found that congenic (N10 on C57B1/6J) D4R-/- mice display a dose-dependent locomotor supersensitivity to MPD. Encouraged by these findings we propose the following course of experimentation: In specific aim 1 we describe studies designed to establish whether acute and/or chronic MPD exposure influences novelty-seeking and impulsivity behaviors in juvenile and adolescent mice lacking D4Rs. In aim 2 we address the question: What are the effects of acute and chronic MPD exposure on gene expression in ADHD-relevant brain regions of wild-type and D4R-deficient juvenile and adolescent mice? Finally, in aim 3, we propose to generate three novel strains of knock-in mice that express either the human allele associated with ADHD (DRD4.7), a "normal" human D4R allele (D4.4), or Green Fluorescent Protein-tagged (GFP) murine D4Rs to more reliably localize the receptor protein in mouse brain tissue. It is our expectation that upon the successful completion of these aims, a better understanding of the complex relationship between MPD exposure, D4R stimulation, gene expression, and rodent behaviors relevant to ADHD will emerge [unreadable] [unreadable]